Nanostructured Conducting Polymer Based Counter Electrodes For Dye-sensitised Solar Cells: An Overview

K.M.H.S. Kiridena1, M.A.C.K. Hansima2, W.S.S. Gunathilake2*

1Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka

2Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka

*Corresponding author:W.S.S. Gunathilake, Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka, 20000, Telephone: 94812394436; E-mail: subhashinig@pdn.ac.lk, smdhgunathilake@yahoo.com

Citation: K.M.H.S. Kiridena, M.A.C.K. Hansima, W.S.S. Gunathilake (2020) Nanostructured Conducting Polymer Based Counter Electrodes For Dye-sensitised Solar Cells: An Overview J Nanosci Res 1(1): 15-26.

Received Date: June 18, 2020; Accepted Date: July 06, 2020; Published Date: July 08, 2020

Abstract

Dye-sensitized solar cells (DSCs) have gained a significant attention as third generation photovoltaic devices due to their high photo-to-electric conversion efficiencies at low production cost. Platinum has been reported as the most felicitous metal for the fabrication of counter electrodes (CEs) in DSCs for its superior electro catalytic activity towards the redox reaction of I3-/I-. Nevertheless, the high cost and susceptibility to corrosion of Pt caused the need of new materials to replace Pt in the CE. Among various discovered materials, conducting polymers with extended conjugate electron system have been widely investigated by researchers. Polyaniline (PANI), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) have drawn much more attention from all of the other conducting polymers as alternative CEs in DSCs due to their high conductivity, electro catalytic activity and simplistic synthesis methods at ambient temperature. These three prominent polymers performed with remarkable photovoltaic performances once they were used as CEs for the DSCs. PEDOT-CE based DSCs showed the highest power conversion efficiency but its cost is high compared to the other two polymers. This review paper discusses the development of PANI, PPy and PEDOT polymers as CE materials with the aid of nanotechnology for the improvement of photovoltaic performances in DSCs.

Keywords: Dye-sensitized solar cell; Counter electrode; Nanotechnology; Polyaniline; Polypyrrole; Poly (3,4-ethylenedioxythiophene); Platinum;

Introduction

The global energy demand and the depletion of readily available fossil fuels are some of the reasons to the search of alternative energy sources, thus renewable solar energy has become crucial. As the largest source of clean energy, the sun deposits 120,000 TW of radiation on the earth [1]. Solar cells are capable of converting solar energy into electrical energy in an efficient way. Initially silicon photovoltaics were used, but the energy requirement for the production of pure Si was extremely high [2]. Hence, the cost of electricity production from commercial silicon photovoltaics were significantly higher than that produced from the fossil fuels. Subsequently O’Regan and Gratzel discovered dye-sensitized solar cells (DSCs) or “Gratzel Cells” in 1991 as an alternative for the Si solar cells [3]. Since then, DSCs have gained more research attention for the past years due to their low production cost, light weight, flexibility, low toxicity, and high efficiency to convert solar energy into electricity [3-6].

Dye-sensitized solar cells:

A conventional DSC is typically composed of a photo anode, a dye (sensitizer), electrolyte and a counter electrode (CE) [7,8]. Photo anode is simply a semiconductor with a wide band gap such as TiO2, ZnO, NiO etc [9]. The most efficient and widely used photo anode is n-type semiconductor TiO2, due to its high charge transport and ability to suppress charge recombination [10]. The photo anodes absorb light in ultra violet region. Dye is an organic compound which is spread over the photo anode which absorbs light in the visible range and produces an electron-hole pair. The excited electron in the conduction band of the dye is then transferred to the conduction band of the TiO2 semiconductor within picoseconds [11]. Afterwards, the electron is transferred to the counter electrode of the cell via an external circuit. Meanwhile, the redox couple donates its electron to the dye and regenerates the dye. Iodide/tri iodide (I-/I3-) is the typical redox couple which is used by the DSC and it is dissolved in a solvent such as acetonitrile.

The oxidized species of electrolyte are reduced at counter electrode. Until the light illumination stops, this operation of the DSC runs as a cyclic process [10]. Following equations from 1 to 6 summarize the above working mechanism of the DSC and figure 1 shows a schematic diagram of a standard DSC [12] (Figure 1).

Figure 1: Schematic diagram of a DSC

Ti O 2 / S + hTi O 2 /S*           1 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGubGaamyAaiaad+eapaWaaSbaaSqaa8qacaaIYaaapaqabaGc peGaai4laiaabccacaWGtbGaaeiiaiabgUcaRiaabccacaWGObWdai abgkziU+qacaWGubGaamyAaiaad+eapaWaaSbaaSqaa8qacaaIYaaa paqabaGcpeGaai4laiaadofacaGGQaGaaiiOaiaacckacaGGGcGaai iOaiaacckacaGGGcGaaiiOaiaacckacaGGGcGaaiiOa8aadaqadaqa a8qacaaIXaaapaGaayjkaiaawMcaaaaa@550B@ Ti O 2 / S * Ti O 2 / S + +  e CB                                  2 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGubGaamyAaiaad+eapaWaaSbaaSqaa8qacaaIYaaapaqabaGc peGaai4laiaadofadaahaaWcbeqaaiaacQcaaaGccqGHsgIRcaWGub GaamyAaiaad+eapaWaaSbaaSqaa8qacaaIYaaapaqabaGcpeGaai4l aiaadofadaahaaWcbeqaaiabgUcaRaaakiabgUcaRiaabccacaWGLb WaaWbaaSqabeaacqGHsislcaWGdbGaamOqaaaak8aadaahaaWcbeqa amaaBaaameaaaeqaaaaakiaabccacaqGGaGaaeiiaiaabccacaqGGa GaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabcca caqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiai aabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGa aeiiaiaabccacaqGGaWaaeWaaeaapeGaaGOmaaWdaiaawIcacaGLPa aaaaa@6103@ Ti O 2 / S + +  e _CB Ti O 2 /S          3 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGubGaamyAaiaad+eadaWgaaWcbaGaaGOmaaqabaGccaGGVaGa am4uamaaCaaaleqabaGaey4kaScaaOGaey4kaSIaaeiiaiaadwgada ahaaWcbeqaaiaac+facaWGdbGaamOqaaaakiabgkziUkaadsfacaWG PbGaam4tamaaBaaaleaacaaIYaaabeaakiaac+cacaWGtbGaaiiOai aacckacaGGGcGaaiiOaiaacckacaGGGcGaaiiOaiaacckacaGGGcGa aiiOaiaacckacaGGGcGaaiiOaiaacckacaGGGcGaaiiOaiaacckaca GGGcWdamaabmaabaGaaG4maaGaayjkaiaawMcaaaaa@5F3E@ Ti O 2 / S +  + 3/2  I _ Ti O 2 / S +1/2  I 3         4 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGubGaamyAaiaad+eadaWgaaWcbaGaaGOmaaqabaGccaGGVaGa am4ua8aadaahaaWcbeqaamaaCaaameqabaGaey4kaScaaaaak8qaca GGGcGaey4kaSIaaeiiaiaaiodacaGGVaGaaGOmaiaabccacaWGjbWd amaaCaaaleqabaWaaWbaaWqabeaacaGGFbaaaaaakiabgkziU+qaca WGubGaamyAaiaad+eadaWgaaWcbaGaaGOmaaqabaGccaGGVaGaaeii aiaadofacaqGGaGaey4kaSIaaGymaiaac+cacaaIYaGaaeiiaiaadM eadaWgaaWcbaGaaG4mamaaCaaameqabaGaeyOeI0caaaWcbeaakiaa cckacaGGGcGaaiiOaiaacckacaqGGaGaaeiiaiaabccacaqGGaGaae iiaiaabccacaqGGaGaaeiiaiaabccacaqGGaWdamaabmaabaWdbiaa isdaa8aacaGLOaGaayzkaaaaaa@6082@ 1/2  I 3 +  e pt 3/2  I                   5 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaaIXaGaai4laiaaikdacaqGGaGaamysamaaBaaaleaacaaIZaWa aWbaaWqabeaacqGHsislaaaaleqaaOGaey4kaSIaaeiiaiaadwgada ahaaWcbeqaaiabgkHiTaaakmaaCaaaleqabaGaamiCaiaadshaaaGc cqGHsgIRcaaIZaGaai4laiaaikdacaqGGaGaamysamaaCaaaleqaba GaeyOeI0caaOGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGa aeiiaiaabccacaqGGaGaaeiia8aadaqadaqaa8qacaaI1aaapaGaay jkaiaawMcaaaaa@5101@ I 3 + 2 e CB 3  I                               6 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGjbWaaSbaaSqaaiaaiodadaahaaadbeqaaiabgkHiTaaaaSqa baGccqGHRaWkcaqGGaGaaGOmaiaadwgadaahaaWcbeqaaiabgkHiTi aadoeacaWGcbaaaOGaeyOKH4QaaG4maiaabccacaWGjbWaaWbaaSqa beaacqGHsislaaGccaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabc cacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeii aiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGa GaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiia8aadaqa daqaa8qacaaI2aaapaGaayjkaiaawMcaaaaa@5905@

Desired properties of a counter electrode

The counter electrode is a vital component in DSCs since it acts as a mediator for collecting electrons from the external circuit, reduces I3- to I- and regenerates the oxidized dye or the sensitizer [13,14]. Therefore a successful CE should consist with properties such as good conductivity, high catalytic activity and electrochemical stability in the electrolyte system. Additionally it should be mechanically stable [1]. Fabrication of CEs from conventional conductive glasses such as indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) is not sufficient since they provide a low catalytic rate for the reduction process. Thus conducting glass substrate must be coated with a catalytic material to accelerate the reaction.

A CE material in a dye-sensitized solar cell should full fill some unique properties such as a low charge transfer resistance (Rct), high surface area, porous nature, optimum thickness, good adhesivity with the transparent conducting oxide (TCO), high reflectance of transmitted light and high exchange current density [15,16]. Platinum has been reported as one of the most appropriate materials to develop CEs in DSCs as it is rich with all the above required properties [17]. However due to the high cost, rarity, susceptibility to corrosion, advance synthetic methods of Pt, a curiosity has been emerged on searching new CE materials such as carbonaceous materials, conducting polymers, transition metal based inorganic materials etc. [18] Among various discovered materials, conducting polymers with extended conjugate electron system have been widely investigated by researchers due to their superior properties [19].

Conducting polymer based counter electrodes

Conducting polymers are organic polymers that possess electrical conductivity ranging from semi-conductors (10-8 S/cm) to metals (105 S/cm) [20]. Conducting polymers are rich with attractive properties such as high conductivity, environmental stability, cost effectiveness, easy synthesis, and electrochemical redox properties [21,22]. Conducting polymers provide conductivity through a conjugated bond system along the polymer backbone, thus it contains continuous sp2 hybridized carbon centers. Two foremost methods to synthesize conducting polymers are chemical oxidative polymerization and electrochemical polymerization [23]. Among them electrochemical polymerization offers unique advantages such as direct deposition of thin film on the substrate, “catalyst free synthetic method”, better adhesivity, higher purity, easier fabrication, controllable surface morphology, in-situpolymerization and uniform thickness of the resultant film [24-26]. These advantages make electrochemical polymerization method as the most worthy one for DSCs applications. Among huge number of discovered conducting polymers up to now, polyaniline (PANI), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) [24], in their nanostructural forms, have drawn much attention as alternative CEs in DSCs due to high conductivity, electro catalytic activity and simplistic synthesis methods at ambient temperature.

There are three main ways to synthesis conducting polymer nanostructures which are,

  1. Templateless synthesis: - The nanostructures are synthesized by selecting the relevant conditions of electrosynthesis at chemically inert electrodes.
  2. Template-assisted synthesis: - Nanostructured templates are first created on the surface of the electrode. Afterwards electropolymerization occurs within the nanosized structural units of a template (such as holes, cavities etc.)
  3. Molecular template-assisted synthesis: - Molecules which contain nanosized cavities are used as templates. They are arranged on the electrode surface, so the electropolymerization proceeds within these cavities resulting nanostructured polymers.

Polyaniline (PANI)

Polyaniline is one of the most intensively studied versatile CE materials when compared with various other conducting polymers. It has appropriate features like easy synthesis, high-conductivity, good environmental stability and interesting redox properties [9, 27, 28]. PANI can be prepared using chemical polymerization or electro polymerization method [29-31]. Many techniques of electro polymerization are used in the synthesis of PANI CEs such as cyclic voltammetry, constant potential, constant current, pulse current, and pulse potentiostatic methods etc. [32-34]. Figure 2 shows the chemical structure of Emaraldine, one of the oxidation forms of PANI (Figure 2).

Figure 2: Structure of Emaraldine

PANI as a CE in DSC has introduced by Li et al. for the first time [35]. Using an aqueous oxidative polymerization reaction with perchloric acid as a dopant in the presence of ammonium persulfate, they synthesized microporous PANI nanoparticles (NPs) with size diameters of 100 nm. The increase of the surface area, lower charge-transfer resistance and higher electro catalytic activity for I- to I3- redox reaction in PANI electrode have resulted an overall energy conversion efficiency of the DSC with 7.15%, which is higher than that of the DSC with Pt counter electrode (6.90%) under the same conditions [35].

The conical structured PANI nanotubes were easily synthesized on indium tin oxide (ITO) conducting poly(ethylene terephthalate)(PET) substrates using electrochemical polymerization method [36]. With the increasing potential from 2.0 V to 2.6 V different PANI structures were obtained such as, dendritic PANI, nanosheets, nanotubes, and nanorods. The novel conical nanotube of PANI was obtained at 2.4 V. This PANI electrode showed excellent charge transport property and high electrochemical activity due to the nanotube structure with high surface area and chemical activity with 0.86% increment in efficiency compared to DSC with Pt counter electrode under the same conditions [36].

PANI nanofiber supported Pt nanoparticle film was prepared by a two-step electrochemical deposition method to use as a counter electrode in DSC and obtained a light-electric energy conversion efficiency of 7.69% which is higher than those with pure PANI (6.58%) or Pt counter electrodes (6.52%) [37].

A flexible counter electrode by electrochemical deposition of PANI nanofibers on graphitized polyimide carbon films for the tri-iodide reduction was synthesized. The photovoltaic device which contains this PANI counter electrode showed the charge transfer resistance measured from the impedance data as 0.5 Ω cm-2 which is lower than that of 1. 8 Ω cm-2 for a conventional platinum electrode and an overall energy conversion efficiency of 6.85% under 1 sun illumination [38].

Another flexible CE was fabricated using a composite of PANI and flexible graphite (FG) in DSC had shown a PCE of 7.36%, which is comparable to 7.87% that of Pt CE [39]. In here FG is used as an alternative to rigid FTO glass substrate by facilitating industrial roll-to-roll applications. Low surface granular diameter (20-50 nm) [39], homogeneous and well- adhered nature of PANI on to FG substrate, enhances the catalytic activity of PANI/FG composite CE which prepared by using an in-situ chemical polymerization.

A CE was prepared using a nanofibrous thin film by depositing as-prepared PANI nanofiber on FTO glass by Marangoni flow [40,41]. Compared with PANI nano-particulate films, PANI nanofibrous films contain high electro catalytic activity as a counter electrode in DSC, due to the formation of an interconnected network structures [42]. The DSC with this PANI CE showed a 3.8% light to electricity conversion efficiency compared with 5.1% by Pt [41].

The electrochemical polymerization method is not applicable for large-scale fabrication. Hence, a simple way to large-scale synthesis of PANI nanobelts by mixing aniline and hydrochloric acid without any templates was introduced [43]. The power conversion efficiency of DSC with PANI nanobelt counter electrode was able to attain 90% of the value of the DSC with Pt counter electrode [43].

A novel CE for the DSC was prepared using graphene/PANI nano composite, which was synthesized by in-situ polymerization. Due to the electrostatic attraction, when aniline monomers are added into graphene dispersion, they can immediately adsorb onto the graphene surface. When ammonium persulfate solution added into the dispersion at 0 °C, aniline monomers adsorbed are initiated to polymerize from the adsorbed sites on the surface, finally resulting PANI nanoparticles with size of 20 nm, on graphene sheet [44]. When a DSC with PANI CE exhibits a power conversion efficiency (PCE) of 4.78% and a fill factor (FF) of 0.54, a graphene/PANI nanocomposite CE exhibits a PCE of 6.09% and a FF of 0.67. This conversion efficiency is comparable to that of the cell with the Pt counter electrode which exhibits a PCE of 6.88 % [44]

Doping of sulfamic acid (SFA) in to uniform polyaniline nanofibers (PANI NFs) has also been studied since still there is a need to work on PANI NFs to enhance its usefulness. A template free interfacial polymerization process has been used and the obtained SFA-doped PANI NFs-based fabricated DSCs have exhibited a power conversion efficiency of 5.5% while the PANI NFs-based DSC demonstrated a PCE of 4.0%. The study states that the enhancement in the PCE was due to the incorporation of SFA into the PANI nanofibers, which gives a higher electro catalytic activity for the redox reaction of I3-/I- [45]. It was found, when SAF is doped, the diameter of PANI NFs has increased about to 40 nm from 30 nm. The doping has caused some aggregation of PANI NFs, which might result in the formation of voids into the fibrous network of PANI NFs and ultimately facilitating an efficient redox reaction of the electrolyte [46].

A unique double-layered PANI deposited ITO substrate has been prepared, having a compact layer of PANI nanoparticles and a layer of PANI nanofibers using cyclic voltammetry method [47]. Due to the increment of electrical resistance of the substrate, the growth mechanism of double-layered PANI is proposed to be the reduction of external current. Light-electricity conversion efficiency was exhibits as 6.58% under a simulated solar illumination with an intensity of 100 mW cm-2, improving the overall efficiency compared to a DSC with Pt counter electrode [47].

In sight of increasing interface area for redox reaction of the electrolyte, polyaniline-single wall carbon nanotube (SWNT) complex, has been combined with negatively charged graphene oxide (GO) as counter electrodes for DSCs. These PANI/SWCNT complex is combined with negatively charged graphene oxide (GO) by a layer-by-layer self-assembly technique. Covalent bonds between PANI and SWNT accelerate the charge transfer. This multilayer structure of (PANI/SWCNT/GO)n (n = bilayer numbers) counter electrode provides a large interface area for high adsorption capacity resulting efficient tri iodide electro catalysis. A successive light-electricity conversion efficiency of 6.88% was able to obtain from a (PANI/4 wt% SWCNT/GO)5 multilayer counter electrode [48].

A composite CE for DSCs prepared using PANI, graphene nanoplatelet (GNP) and multi-wall carbon nanotube (MWCNT) deposited on FTO substrate had shown a PCE of 7.45%, which is comparable with that of 7.62% of Pt CE [49]. In here GNPs and MWCNTs have been used as additives to increase conductivity and incorporation of them into PANI lowers the sheet resistance by facilitating high charge transfer [49].

A CE prepared using a nanocomposite of graphene nanosheet (GNS) and PANI which was anchored by NiS had shown a PCE of 5.36% which is comparable to 5.97% of PCE Pt CE [50]. PANI improves the catalytic activity by restricting the aggregation of NiS and GNS on electrodes [50]. This CE was prepared using a paste of NiS/GNS/PANI deposited on FTO glass.

A CE prepared using Nickel nanoparticles, PANI and graphene to replace Pt CE for DSCs had shown a PCE of 5.80%, which is higher than that to Pt CE (5.30%) [51]. In here high PCE is due to the improvement of electro catalytic activity of PANI due to high surface area of Ni-nanoparticles and high electrical conductivity of graphene [51].

Table 1 summarizes the photovoltaic performances of PANI obtained from different methods as CE.

Many research work have been conducted to replace CE from pure PANI nanostructures as wellas PANI nono composites [52-56]. While considering the above findings, it emphasizes that still there is a need of enhancing the efficiency of a PANI CE. More research work should be conducted to make PANI composites that not only increase the conductivity but also that can act as a pore former, which can enhance the surface area, thus increase the catalytic activity of the CE (Table 1).

Polypyrrole(PPy)

Table 1: Photovoltaic performances of different morphologies of PANI as counter electrodes for DSC

Nature of PANI

Method of Preparation

Jsc (mA cm-2)

Voc (V)

FF

PCE (%)

Ref

Microporous PANI

Oxidative Polymerization

14.6

0.714

0.69

7.15

[35]

PANI FG composite

In situ Chemical Polymerization

15.41

0.657

0.73

7.36

[39]

Nano fibrous PANI

Maragoni Flow Method

8.9

0.662

0.53

3.8

[41]

PANI nanobelts

Template Free - Chemical Polymerization

13.22

0.714

0.66

6.32

[43]

rGO - PANI nano particles

Chemical Oxidation Polymerization

12.88

0.74

0.65

6.15

[57]

Graphen-PANI nanocomposite

In Situ Oxidative Polymerization

13.28

0.685

0.67

6.09

[44]

Salfamic acid doped PANI NFs

Interfacial Polymerization

13.62

0.74

0.53

5.5

[45]

Bilayer PANI

Cyclic Voltammetry Method

13.4

0.728

0.68

6.58

[47]

(PANI/SWCNT/GO)n

Chemical Polymerization

-

-

-

6.88

[48]

PANI/GNP/MWCNT

Chemical/Electrochemical Deposition Method

17.51

0.734

-

7.45

[49]

NiS/GNS/PANI

Chemical Oxidation Polymerization

12.13

0.67

0.66

5.36

[50]

Ni PANI G nanocomposite

Chemical Oxidation Polymerization

13.43

0.745

0.85

5.8

[51]

Another conducting polymer which was most determinedly studied is polypyrrole (PPy). Basically, it is due to the high polymerization yield, high conductivity, better chemical and mechanical stability, good catalytic activity, successive environmental stability and thermal stability in air up to 150 oC [58-61]. However, when compared with PANI based CEs in DSCs, PPy based CEs yield relatively lower efficiencies due to lower conductivity and high charge transfer resistance in PPy. By using proper dopants, utilizing surface morphologies and synthetic techniques, the efficiency can be improved. Figure 3 shows the chemical structure of PPy (Figure 3).

Figure 3: The structure of Polypyrrole

CEs in DSCs have been replaced from PPy nanostructures in various ways [62-66]. As an effort on replacing Pt CE in DSCs from PPy polymer, PPy nanoparticles were synthesized and coated on a FTO glass. When these porous PPy nanoparticles, with diameter of 40-60 nm, covered on the FTO glass and used as a CE, resulted an overall energy conversion efficiency in DSC with 7.66%. This amount is higher (11%) than that of the DSC with Pt CE under the similar conditions [67]. PPy nanoparticles propound good adhesion to substrate, high surface area and small charge transfer resistance and offer a higher electro catalytic activity in PPy electrode.

Furthermore, various efforts have been taken to improve the overall performance of DSCs, such as modifying the morphology of PPy nanoparticles when using PPy polymer as a CE material. Accordingly three kinds of PPy nanoparticles with various morphologies were synthesized by incorporation of a anionic-type surfactant (docusate sodium salt), a cationic type surfactant (acetylmethyl ammonium bromide), and an amphiphilic-type polymer (poly(oxyethylene)9 methacrylate)) resulting irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), respectively [68]. The PPy-HNS film confirmed its largest BET surface area of 61.18 m2 g-1 than the PPy-IS films (34.80 m2 g-1) and PPy-NS (58.42 m2 g-1).

The PPy-HNS based CE showed the highest electro catalytic ability for the reduction of I3- and the lowest charge transfer resistance, as compared to PPy-IS and PPy-NS [68].

The rigidity of FTO glass restricts the development of a flexible CE. Therefore a study has been carried out in order to develop free-standing polypyrrole nanotube films, which can be used as a CE to replace both the Pt and fluorine doped tin oxide (FTO) glass in dye-sensitized solar cells. PPy nanotubes were formed using self-degraded template method [69]. The DSCs produced with these free standing PPy membranes have performed an excellent power conversion efficiency of 5.27%, which is about 84% of the cell with a standard Pt/FTO CE (6.25%) [69].

Monodisperse and spherical PPy nanoparticles (size of 85 nm) have been easily synthesized within micelle template (MTAB) via chemical oxidative polymerization (using FeCl3as the oxidizing agent) to employ as a counter electrode to the DSC. This micelle polymerization technique can be used in large scale production of PPy nanoparticles [70]. HCl vapor treatment which was carried out as a post doping to the PPy layer decreased the surface resistivity. Moreover it allowed the electrons to move into the PPy layer efficiently and participate in the electro catalytic reaction of the redox couple. By controlling the thickness and HCl vapor treatment time, the DSCs made of HCl-doped PPy/FTO CEs exhibited power conversion efficiencies of 7.73% which is comparable to the value (8.2%) using conventional Pt CEs [70]. Furthermore ultrathin polypyrrole nanosheets (UPNSs) were synthesized by chemical oxidation via organic single-crystal surface-induced polymerization (OCSP) using sodium decylsulfonate (SDSn) as a template to use as CEs [71]. Dye-sensitized solar cells constructed using HCl-doped UPNS CEs resulted a power conversion efficiency 6.8%, which is comparable to that of Pt CE-based DSCs (7.8%) [71]. These UPNS have showed alike morphology when compared to graphene sheets with higher surface area and active sites.

Nano graphite/polypyrrole (NG/PPy) composite film was prepared via in-situ polymerization on FTO glass and performed as CE in DSCs [72]. Doping of nanographite into PPy has resulted in high-efficient catalytic activity due to the fast mass transport at the interface of electrolyte-electrode. Ultimately, a high power conversion efficiency of 7.40% has been recorded under 1 sun illumination compared to a standard Pt electrode based DSC (6.85%) [72].

Furthermore PPy/rGO (polypyrrole/reduced grapheme oxide) composite as a CE material for DSCs has been studied. This composite was fabricated using a two-step electrochemical process. A DSC with PPy/rGO composite based CE has resulted with a power conversion efficiency of 6.45%, which is comparable to a Pt based DSC (7.14%) [73].

In-situ reduction was done to obtain reduced graphene oxide (rGO)/PPy composite on FTO glass as CE for DSCs. rGO/ PPy CE had shown a PCE of 8.14%, which is comparable with that to Pt CE (8.34%) [74]. In here rGO sheets act as conductive channels and co-catalyst for the CE of DSCs [74].

Graphene quantum dot (GQD)-doped polypyrrole (PPy) also has been reported as a CE in DSCs. The porous structure of GQDs-doped PPy caused for a high catalytic activity than densely structured normal PPy. Therefore the cell showed an electricity conversion efficiency of 5.27% which is close to the efficiency of platinum CE in DSC [75]. Polypyrrole-cobalt-carbon (PPY-Co-C) nano composites also has been tried out as a CE material for DSCs. Reduction of I3-is supported with large inner surface area and homogeneously distributed Co active sites. The cell performed a power conversion efficiency of 6.01% is comparable to standard Pt CE based DSCs (6.61%) [76].

A complex of PPy-single wall carbon nanotube (SWCNT) synthesized using reflux techniques and subsequent in-situ polymerization had yielded an enhanced PCE of 8.30% [77]. Formation of covalent bonds within PPy-SWCNT complexes accelerate the charge transfer and enhance the photovoltaic performances in DSCs due to long-range ordering and good electron dislocation in their structure [77].

A composite film of multiwall carbon nanotubes (MWCNTs) and PPy on FTO substrate had shown PCE of 7.41%, which is higher than that of sputtered Pt (6.85%) on FTO substrate as CE in DSCs [78]. Uniform distribution of MWCNT on the PPy nanoparticles has enhanced the conductivity and the electro catalytic activity by forming a transport network like structure [78].

A nanocomposite of silver-PPy-multiwall carbon nanotube (MWCNT) synthesized via electro deposition method on stainless steel substrate had displayed an acceptable PCE of 7.0% with re-spect to thermally deposited Pt electrode 7.6% [79]. This enhanced conductivity caused by the increase of average localization length due to composite π conjugated structure of MWCNTs and quinoid rings of polymer chains. Silver nanoparticles allow the formation of well ordered structure as well as larger surface area for reduction of electrolyte [79].

Novel CE made by using a composite film of graphene-coated alumina (GCA) and PPy coated on FTO substrate exhibited an efficiency of 7.33% with respect to 7.57% Pt CE [80]. Al2O3was added to prevent sheet aggregation of graphene oxide. Therefore, GCA had shown greatly improved catalytic activity of PPy CE and hence improved the photovoltaic performances of DSCs [80]. Table 2 summarizes the photovoltaic performances of PPy obtained from different methods as CE (Table 2).

Poly(3,4-ethylenedioxythiophene) (PEDOT)

In 1988, Jonas et al. developed poly(3, 4-ethylenedioxythiophene) (PEDOT), an electronically conducting polymer, which contains desirable properties such as high room temperature conductivity, transparency and remarkable stabilities [82, 83]. PEDOT yields the highest efficiency and electro catalytic activity towards I3-/I- in Pt free DSC, when compared with PANI and PPy. PEDOT exhibits an

Table 2: Photovoltaic performances of different forms of PPy as counter electrodes for DSC

Nature of PPy

Method of Preparation

Jsc (mAcm-2)

Voc (V)

FF

PCE (%)

Ref

PPy film

Electropolymerization

9.66

-

0.66

5.1

[81]

PPy nanoparticles

Synthetic Chemical Method

15.01

0.74

0.66

7.66

[67]

PPy IS (Irregular sheet)

Chemical Oxidative Polymerization

11.52

0.7

0.67

5.46

[68]

PPy HNS ( Hierarchical nanosphere)

Chemical Oxidative Polymerization

16.49

0.7

0.58

6.71

[68]

PPy NS (Nanosphere)

Chemical Oxidative Polymerization

14.1

0.7

0.63

6.31

[68]

PPy nanotube (free ITO)

Self-assembly

13.1

0.716

0.56

5.27

[69]

Spherical PPy

Chemical Oxidative Polymerization

15.5

0.791

0.64

7.73

[70]

PPy UPNSs

Chemical Oxidative polymerization

15.2

0.721

0.62

6.8

[71]

Nano graphite/ PPy

In Situ Electro deposition

14.83

0.765

0.65

7.4

[72]

PPy/rGO

Electro Chemical Polymerization

15.48

0.695

0.6

6.45

[73]

PPy/rGO

Chemical Oxidative Polymerization

15.81

0.724

0.71

8.14

[74]

PPy SWNT complex

Reflux Process

15.68

0.742

0.71

8.3

[77]

MWCNT/Ppy composite

Electrochemical Polymerization

14.83

0.77

0.65

7.42

[78]

Ag Ppy MWCNT nanocomposite

Electro Deposition

13.95

0.798

0.67

7

[79]

PPy/GCA

Electrochemical Polymerization

16.54

0.717

0.62

7.33

[80]

excellent conductivity (300-500 S cm-1) [84], which is higher than those of PANI (0.1-5 S cm-1) [85] and PPy (10-50 S cm-1) [86]. CE based on PEDOT-nanoparticles [87, 88] PEDOT-carbon nanotubes [89,90], PEDOT-nano fibers [91], PEDOT-nano composites [92,93, 94] etc have been investigated [95,96]. Figure 4 shows the chemical structure of PEDOT (Figure 4).

Figure 4: Structure of PEDOT

The initial investigation on fabrication of PEDOT CEs by chemical polymerization was published in 2002 [97]. The PEDOT was doped with TsO-(ptoluenesulfonate) and poly(styrenesulfonate) (PSS). Potential current efficiency of PEDOT-Tso was better than PEDOT-PSS due to the exposer of PSS-that resulted in increased over potential [97]. In cyclic voltamograme analysis the oxidation/reduction peaks of I-/I3- observed on the PEDOT-TsO electrode were indistinguishable to the behavior of the Pt electrode. However, in the PEDOT-PSS electrode, the oxidation current was remarkably decreased. The author explains this as the reduction of the redox active site of the PEDOT-PSS film due to the exposure of PSS, which prevent the approach of I- or I3- to the active site of the PEDOT chain [97]. Also when the thickness of the PEDOT-TsO increased, the change transfer resistance has increased.

The performance of PEDOT CEs can also be improved by introducing nanostructures. A study on application of nanotubes (NT) of PEDOT CE has been carried out in order to prove this. It has confirmed that the performance of PEDOT-NT CE efficiency (8.3%) is better than a flat PEDOT CE (7.9%) due to their high effective area and that performance is superior as the conventional platinized counter electrodes (8.5%). PEDOT has been electropolymerized onto ZnO Nanowire array templates (working electrode) in a three-electrode electrochemical cell [98].

In addition PEDOT nanofibers were prepared by chemical oxidative polymerization with the aim of obtaining low surface resistance, good adhesion and increased catalytic activity. The PEDOT-NFs with 10-50 nm diameters was prepared from cylindrical micelles formed by sodium dodecyl sulfate (SDS) and FeCl3. The conductivity of PEDOT-NF was as high as 83 S cm-1, while that of bulk PEDOT was 0.5 S cm-1 and the PEDOT-NF formed a porous network structure of CEs, which facilitated penetration of the electrolyte for redox reactions. Ultimately, CEs prepared from nanofibers, in DSCs showed an excellent PCE of 9.2% while PEDOT alone showed a PCE of 6.8% [99].

A low-cost nanopatterned highly conductive PEDOT film was synthesized on a flexible plastic substrate using a chemical polymerization method. This was used as Pt, TCO-free CE for DSCs. This exhibited a light-current efficiency of 7.1%, while a reference device with a Pt/FTO paired CE resulted an efficiency of 7.6% [100]. The nanopatterning of the polymer film can increase the number of reactive sites in the CE, hence the catalytic activity also increased [100].

Carbon nanotubes (CNTs) also contain impressive properties, such as high mechanical strength, thermal stability and impressive electronic properties [101]. Therefore CNTs can be used in many electrical devices. Accordingly incorporation of multiwall carbon nanotubes (MWCTNs) as the CE to DSCs have been studied. Composite films of PEDOT and functionalized, multi-walled, carbon nanotubes (PEDOT-MWCNT) were synthesized on a FTO glass substrate by electro polymerization technique. Due to the high porous structure, the power conversion efficiency of the DSC with a PEDOT-MWCNT CE (efficiency 5.47%) was 13.0% higher than when compared with a PEDOT CE (5.43%) using the same conditions due to the high porous structure [102].

For further improvement of the DSCs Ouyang and coworkers explored the combination between PEDOT:PSS and MWCNT as a composite CE for DSCs for the first time. Film was fabricated from spin coating the aqueous solution of the MWCNT/PEDOT:PSS composite. The device exhibited excellent performance with a power conversion efficiency of 6.5%. This performance is close to the DSCs with Pt CEs [103].

Another interesting study has been carried out on PEDOT films incorporating gold nanoparticles (AuNPs) as CE in DSCs. AuNPs were synthesized according a well-known Turkevich method [104]. With Co(III)/Co(II) based redox couple, the power conversion efficiency has been increased to about 130% compared to the use of PEDOT alone. It was found the efficiency of the solar cell with Pt CE is 1.3%, PEDOT CE is 1.4% and AuNPs/ PEDOT is 3.2% [104].

PEDOT films containing nano-meadows morphology has been prepared by pulse potentiostatic method on FTO plates. It resulted a better electro catalytic activity for the redox reaction than that with Pt CE. It was proved by the cathodic peak current in cyclic voltammogram measurements. The DSC with the nano-meadow PEDOT CE exhibited photo voltaic conversion of 6.40%, which is higher than a DSC with a Pt CE (5.75%) [105].

Moreover a CE has been fabricated from PEDOT/PSS composite deposited on Ag network (Ag NW) substrate. The electrode resulted the electron generation at both sides on illumination. Furthermore PEDOT/PSS layer has been doped with 20% TiO2 nanoparticles for the enhancement of the cell performance. In order to prevent the Ag NW corrosion by the iodine electrolyte, the Ag NW substrate was covered with a PEDOT/PSS layer. The performance of the DSC was 5.13% in efficiency, which is very similar to those of cells using a platinum counter electrode (5.36%) [106].

In another attempt, PEDOT coated graphene has been used as the CE in DSC to replace both the Pt and TCO. Surface resistance showed a decrement due to the addition of high conductive graphene in PEDOT film. A power conversion efficiency of 6.26% has been resulted [107]. In addition, PSS in PEDOT together with graphene has been synthesized by spin coating in order to replace Pt on CE. A power conversion efficiency of 4.66% has been reported from the cell in comparison with 5.95% conversion efficiency shown by cells containing platinum CE [108].

Reduced grapheme oxide (rGO)/PPy/PEDOT thin film was synthesized to deposit on FTO glass and performed as a CE in DSC. Power conversion efficiency was 7.1% due to the effect of conducting polymer and graphene. The efficiency is comparable to that of Pt-based cells made under similar Conditions [109].

PEDOT was electrochemically deposited on porous hard template film of TiC nanoparticles (50 nm) to fabricate TiC-PEDOT CE for DSCs, which had shown the efficiency of 8.09% to Co(bpy)2+/3+ redox couple [110]. On the other hand, a TiN-PEDOT CE had shown even better efficiency of 8.26% for the same redox couple. An interconnected conductive network between TiN or TiC nanoparticles with PEDOT facilitate better electron transport properties. Another article reported a PCE of 7.06% for CE fabricated with TiN-PEDOT:PSS for I-/I3- redox couple [111].

An inorganic/organic nanocomposite film composed of Si nanoparticles (Si-NPs) and PEDOT:PSS obtained from a mechanical way used as the CE of DSCs had a PCE of 5.7% and a FF of 0.51 [111], both in which are higher than that of PEDOT:PSS CE. Due to the increased electrochemical surface with the addition of Si-NPs led to enhance those photovoltaic parameters for Si-NPs/PEDOT:PSS CE. This novel CE had shown a comparable PCE with Pt CE (6.6%) [112] for DSCs.

Both PEDOT and PPy are promising candidates to make Pt free CEs for DSCs. A composite film of PEDOT:PSS and PPy fabricated using an electrochemical polymerization method had reached a PCE of 7.60% which is comparable with Pt sputtered CE (7.73% ) for DSCs [113]. A uniform porous film structure had resulted from PEDOT:PSS and PPy particles are expected to facilitate an effective electro catalytic surface to ensure higher PCEs for DSCs.

Table 3 summarizes the photovoltaic performances of different forms of PEDOT obtained from different methods as CE for DSC (Table 3).

Summary

DSCs, as a power generation device, have gained more research attention for the past years due to their low production cost and high efficiency to convert solar energy into electricity. Counter electrode plays a major role in this device and Pt has been reported as the most appropriate material to develop CEs in DSCs. However, due to the high cost, susceptibility to corrosion,

Table 3: Photovoltaic performances of different forms of PEDOT obtained from different methods as counter electrodes for DSC

Nature of PEDOT

Method of Preparation

JSC(mA cm-2)

VOC(V)

FF

PCE (%)

Ref

PEDOT:PSS

Chemical Polymerization

11.2

0.67

0.61

4.6

[95]

PEDOT nanotube

Template Assisted Electro Polymerization

16.24

0.72

0.7

8.3

[56]

PEDOT flat film

Electro Polymerization

15.38

0.73

0.69

7.9

[56]

PEDOT nanofiber

Template Assisted Chemical Oxidative Polymerization

17.5

0.72

0.73

9.2

[96]

PEDOT/MWCNT

Electro Polymerization

11.5

0.74

0.64

5.47

[99]

PEDOT-Au nanoparticles

Electro Polymerization

7.6

0.71

0.6

3.2

[84]

PEDOT:PSS/MWCNT

Spin Coating

15.5

0.66

0.63

6.26

[103]

PEDOT:PSS-Graphene

Spin Coating

12.96

0.72

0.48

4.66

[104]

RGO-PEDOT:PSS

Spin Coating

17

0.75

0.55

7.1

[105]

TiN-PEDOT for Co2+/3+ redox couple

Electro Polymerization

13.21

0.84

0.75

8.26

[107]

TiC-PEDOT for Co2+/3+ redox couple

Electro Polymerization

12.68

0.84

0.76

8.09

[107]

TiN-PEDOT: PSS for I- / I3- redox couple

Mechanical Mixture

14.45

0.73

0.67

7.06

[108]

PEDOT: PSS/PPy

Electro Polymerization

14.27

0.75

0.71

7.6

[109]

advance synthetic methods of Pt, the need of alternative materials to replace Pt is crucial. Among various discovered materials, conducting polymers with extended conjugate electron system have been widely investigated by researchers due to their superior properties. PANI, PPy and PEDOT polymer nanostructures and hybrid composites have been found as cost effective CE materials in Pt free DSCs. PANI has appropriate features like easy synthesis, high-conductivity, good environmental stability and interesting redox properties to replace Pt. PPy based CEs in DSCs perform with lower efficiencies compared to PANI, due to low conductivity and high charge transfer resistance. PEDOT-CE based DSCs performed the highest efficiency compared to PANI and PPy with the highest conductivity and electro catalytic activity towards I3-/I- among the three polymers considered in this review. When the morphologies of these polymers are developed with various nanostructures and when the polymers make hybrid nano composites they earn high surface area:volume ratio which leads to a high level of reactivity. Thus CEs in DSCs exhibit enhanced efficiencies. All the above discussed methods are cost effective and therefore more research work should be carried out in order to investigate nanostructure polymer based novel CEs for DSCs in future.

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